The present invention relates generally to the combustion of nitrogen containing carbonaceous solids such as oil shale, spent shale, coal and tar sands. More specifically, the present invention relates to the combustion of carbon residues present in spent shale produced during production of shale oil from oil shale.
Oil shale is a Marlstone-type inorganic material mixed with an organic polymer called kerogen. The three major aspects of shale oil production are mining, crushing and retorting. The present invention has application to processes for retorting oil shale. As is well-known, kerogen in its natural form is not suitable for commercial use. Oil shale is typically pyrolyzed or retorted at elevated temperatures to decompose the kerogen to volatile product oils and gases. These pyrolysis product vapors are removed from the retort for condensation and fractionation.
The solid residue of processed oil shale remaining after pyrolysis is commonly referred to as spent shale. Spent shale typically will include a combustible carbonaceous residue in amounts ranging up to approximately 10 weight % and higher depending upon the source of the oil shale and the retorting conditions. This carbonaceous residue will typically contain from 0.2 weight percent to 0.8 weight percent nitrogen. The residue is usually distributed throughout the entire spent shale particle.
In order to prevent this combustible material from being discarded as waste, the combustible residue is combusted in a spent shale combustor at temperatures above pyrolysis conditions usually in the range of 1100.degree. F. to 1700.degree. F.. The heat generated by combustion of the carbonaceous residue is extracted from the combustor and may be utilized for various heat requirements throughout the process. The final residue remaining after spent shale combustion will usuallly contain less than 1% by weight carbon residue and is commonly referred to as shale ash.
In many pyrolysis processes, heat carrying bodies are added to the retort to provide the necessary heating for kerogen decomposition. In many processes utilizing heat carriers, the heat carriers are separated from the spent shale prior to combustion. In others, the heat carriers are passed along with the spent shale as a solids blend to the combustor where they are reheated during combustion of the spent shale.
When combusting carbonaceous fuels containing nitrogen compounds, such as spent shale, it is desirable to achieve essentially complete combustion of the fuel content of the solid. To achieve the desired complete combustion, an oxidizing atmosphere which includes an excess of oxygen must be utilized. However, when combustion is carried out in such an oxidizing atmosphere, the nitrogen compounds present in the solids are oxidized to nitric oxide which is an undesireable polutant. If oxygen levels are reduced so that combustion is carried out in any oxygen deficient or reducing atmosphere, nitric oxide formation is inhibited, but fuel combustion is less complete. This results in carbon monoxide and hydrocarbons being carried out of the combustor in the combustor flue gas and an attendant loss in fuel combustion efficiency.
It would therefore be desirable to provide a combustion process or system which achieves complete fuel combustion with minimal production of undesirable nitric oxide.